Answer: (n 1 -1)t 1 - (n 2 -1)t 2 divided by λ (in terms of fringes).
- A (n<sub>1</sub>-1)t<sub>1</sub> - (n<sub>2</sub>-1)t<sub>2</sub> divided by λ (in terms of fringes)
- B (t<sub>1</sub>-t<sub>2</sub>)/λ, omitting the refractive indices entirely from the path difference
- C (n<sub>1</sub> t<sub>1</sub> - n<sub>2</sub> t<sub>2</sub>)/λ, omitting the subtraction of unity from each index
- D (n<sub>1</sub>+n<sub>2</sub>)(t<sub>1</sub>-t<sub>2</sub>)/λ, an incorrect combination of the indices and thicknesses
Correct answer: A. (n<sub>1</sub>-1)t<sub>1</sub> - (n<sub>2</sub>-1)t<sub>2</sub> divided by λ (in terms of fringes)
Explanation: Extra optical path by slab 1: (n<sub>1</sub>-1)t<sub>1</sub>. Slab 2: (n<sub>2</sub>-1)t<sub>2</sub>. Net extra path difference: (n<sub>1</sub>-1)t<sub>1</sub> - (n<sub>2</sub>-1)t<sub>2</sub>. Fringe shift = this net extra path / λ (toward slit 1 if slab 1 has more path).
Two coherent slits S₁ and S₂ act as secondary sources; at each point on the screen, the path difference between light from S₁ and S₂ determines whether the waves arrive in phase (bright fringe) or out of phase (dark fringe), producing the characteristic alternating fringe pattern.
Concept context
Huygens principle, Young's double slit, diffraction, polarization. Essential for JEE.